Ignition circuit for an arc discharge lamp



May 5, 1970 T. N. TYLER 3,510,725

IGNITION CIRCUIT FOR AN ARC DISCHARGE LAMP Filed Dec. 16, 1968 INVENTOR. TOMMY N. TYLER ATTORNEY.

United States Patent M 3,510,725 IGNITION CIRCUIT FoR AN ARC DISCHARGE LAMP Tommy N. Tyler, Littleton, Colo., assignor to Honeywell Inc., Minneapolis, Minn., a corporation of Delaware Filed Dec. 16, 1968, Ser. No. 784,010 Int. Cl. H051) 41/14 US. Cl. 315-183 3 Claims ABSTRACT OF THE DISCLOSURE An ignition circuit is disclosed wherein an arch discharge lamp is series-connected with a circuit-limiting resistor between the terminals of a DC. power supply. Electrical means responsive to the lamp voltage drop are included which are operative to generate spark discharges across a spark gap whenever the lamp is extinguished. The spark gap is connected across the electrodes of the arch discharge lamp so that the lamp is ignited or reignited in response to the spark discharges.

The present invention relates to ignition circuits, and more particularly to an improved ignition circuit suitable for use as a lamp start circuit. Such ignition circuits are used, for example, in direct recording instruments as lamp start circuits for igniting and reigniting arc discharge lamps, such as high pressure mercury vapor lamps. The ignited mercury vapor lamps provide ultraviolet light beams which are employed as recording elements in these instruments.

Ignition circuits have, heretofore, been developed which ignite or reignite an arch discharge lamp by generating and impressing a series of high potential spark discharges across the electrodes of such a lamp. These ignition circuits have typically included a vibrator having a mechanical switching arrangement. The vibrator operated in these circuits to control the generation of the spark discharges by rapidly closing and opening the switching arrangement whenever the arc discharge lamp was extinguished. Once the lamp was ignited the vibrator would cease to close and open the switching arrangement, thereby ceasing the generation of spark discharges. Unwanted spark discharges detrimental to the life of the lamp electrodes, however, were often generated immediately subsequent to the ignition of the lamp since the mechanical switching arrangement tended to coast to a stop when deenergized, rather than coming to an instant halt. The use of vibrators in such ignition circuits had the further disadvantages that the vibrators were bulky, consumed relatively large amounts of power, and had mechanical moving contacts which tended to stick, thereby reducing the overall reliability of the ignition circuits.

It is, accordingly, an object of the present invention to provide an improved ignition circuit suitable for use as a lamp start circuit which obviates the disadvantages of the prior art circuits.

It is further an object of the present invention to provide an ignition circuit as set forth which is characterized by being operative to initiate conduction in an are discharge lamp at the first switching on of the lamp as well as reinitiating conduction in the lamp after brief extinguishments.

In accomplishing these and other objects, there has been provided, in accordance with the present invention, an arc discharge lamp connected in series with a currentlimiting resistor across the terminals of a DC. power supply. A voltage drop appears across the arc discharge lamp which is of a first magnitude whenever the lamp is conductive and of a second greater magnitude whenever the lamp is nonconductive. Electrical means responsive 3,510,725 Patented May 5, 1970 to the lamp voltage drop are provided which are operative to generate ignition pulses whenever the lamp is extinguished and its voltage drop increases to the second magnitude. The ignition pulses when generated produces spark discharges across a spark gap. The spark gap is connected across the electrodes of the arc discharge lamp so that whenever spark discharges are produced, conduction will be initiated in the then extinguished lamp. Thereby, the arc discharge lamp will be ignited or reignited.

A better understanding of the invention may be had from the following detailed description when read in connection with the accompanying drawing in which:

The single figure drawing is a schematic diagram of an ignition circuit according to the present invention.

Referring to the drawing in more detail, there is shown therein a power supply transformer 2. The transformer 2 has a primary winding defined by terminals 3 and 4, and a secondary winding defined by terminals 5 and 6. The secondary winding also has an intermediate terminal 7. Connected across the transformer terminals 5 and 7 are the input terminals of a conventional diode bridge 8. Leads 9 and 10 are connected to the output terminals of the bridge 8, the lead 9 being connected to the output terminals of relative positive polarity. The lead 10 is connected to the bridge output terminal of relative negative polarity and provides a bus for carrying the common reference potential. A filter capacitor 15 is connected between the leads 9 and 10. One terminal of a resistor 16 is connected to the lead 9 while its other terminal is connected to the anode of an arc discharge lamp 17. The cathode of the lamp 17 is connected to the lead 10.

Connected in parallel with the lamp 17 is a series-connected resistor 30 and capacitor 31. The resistor 30 is connected to the anode of the lamp 17, and the capacitor 31 is connected to the lead 10. The junction point between the resistor 30 and the capacitor 31 is connected to the control electrode of a silicon control rectifier (SCR) 23 through a series-connected symmetrical Zener diode 32 and a current limiting resistor 33. One terminal of a capacitor 22 is connected to the anode of the SCR 23. The other terminal of the capacitor 22 is connected to a terminal 18 of a pulse transformer 27. The terminal 18 and a terminal 19 define the primary winding of the transformer 27, the terminal 19 being connected to the lead 10. The secondary winding of the transformer 27 is defined by terminals 20 and 21. The terminal 21 is also connected to the lead 10. A spark gap 34 is connected between the terminal 20 and the anode electrode of the lamp 17. The term spark gap, as here used indicates a device having .two high potential electrodes with a space between through which may be passed spark discharges of electricity. The terminal 6 of the power supply transformer 2 is connected through a series-connected diode 25 and resistor 24 to the junction point between the capacitor 22 and the anode of the SCR 23. The diode 25 is biased so that the capacitor 22 will be charged whenever the potential on the terminal 6 is positive with respect to the potential on the lead 10. The cathode of the SCR 23 is connected to the lead 10.

To illustrate the operation of the exemplary circuit, it is assumed that initially the arc discharge lamp 17 is nonconductive and that a constant A.C. voltage of suitable magnitude has just been applied to the power supply transformer terminals 3 and 4. Thereby, A.C. signals are induced in the secondary winding of the transformer 2 and appear upon the transformer terminals 5, 6 and 7. The A.C. signals appearing across the terminals 5 and 7 are fully-wave rectified by the bridge 8 to produce a DC. output voltage across the bridge output terminals. The DC. output of the bridge 8 is then filtered by capacitor 15 to remove the ripple components therefrom. Thus, a substantially smooth D.C. potential diiference of a predetermined magnitude is established between the leads 9 and 10. The magnitude of this potential diiference is predetermined, for reasons hereinafter explained, to have a value greater than that of the breakdown voltage of the symmetrical Zener or trigger diode 32.

Since the lamp 17 is, at the instant, nonconductive, it acts as an open circuit. The voltage drop across the lamp electrodes, i.e., its open circuit voltage drop, therefore substantially equals the potential difference between the leads 9 and 10. As a result, the capacitor 31 is charged through the resistor 30 towards this open circuit voltage drop. The charging rate is determined by the time constant provided by the resistor 30 and the capacitor 31. In actuality, the capacior 31 only charges to a voltage slightly in excess of the breakdown voltage of the trigger diode 32. At that instant, as below discussed, the capacitor 31 will be discharged through the trigger diode 32, the current limiting resistor 33 and the control electrodecathode path of the SCR 23.

Simultaneous with charging the capacitor 31, the AC. singles appearing across the transformer terminals 6 and 7 are half-wave rectified by the diode 25 and one of the diodes of the bridge 8. Thereby, a pulsating D.C. signal is produced for charging the capacitor 22 to the peak voltage appearing between the terminals 6 and 7, e.g., such as 160 volts. This voltage to which the capacitor 22 is charged is determined to be of suflicient magnitude for inducing an ignition pulse in the secondary winding of the transformer 27 which is effective to initiate conduction in the lamp 17. Further, the time constant of the R-C circuit comprised of the resistor 24 and the capacitor 22 is smaller than that provided by the circuit comprised of the resistor 30 and the capacitor 31 so that the capacitor 22 is fully charged or recharged before the charge on the capacitor 31 exceeds the breakdown voltage of the trigger diode 32.

When the capacitor 22 becomes fully charged, the capacitor 31 continues charging until the breakdown voltage of the trigger diode 32 is exceeded. At that instant, the diode 32 conducts and the charge on the capacitor 31 is transmitted through the diode 32 and the resistor 33 to the control electrode of the SCR 23. The SCR 23, which functions as a voltage actuated switch, is thereby made conductive and provides a discharge path which short-circuits the current path across the capacitor 22 and the primary winding of the transformer 27. Thereby, the capacitor 22 is rapidly discharged through the current path of the SCR 23 and the primary winding of the transformer 27. The rapid discharge through the primary winding of the transformer 27, in turn, induces an ignition pulse in the transformer secondary winding. This ignition pulse correspondingly causes a spark discharge to occur across the high potential electrodes of the spark gap 34. The energy of the spark discharge is impressed across the electrodes of the lamp 17, thereby initiating conduction in the lamp 17, lighting the lamp 17. During the rapid discharge of capacitor 22 through SCR 23, a resonant L-C circuit is formed with the primary winding of transformer 27. This resonant circuit causes the potential across SCR 23 to reverse polarity upon completion of the discharge of capacitor 22, thus turning off SCR 23 after each discharge of capacitor 22. It is possible that several discharges of the capacitor 22 might be required to ignite the lamp 17 With the lamp 17 ignited, the potential difference between the leads 9 and is sufficient to maintain ignition. A direct current then flows through the current limiting resistor 16 and the lamp 17, and the voltage difference between the leads 9 and 10 is divided proportionally between the resistor 16 and the lamp 17 The voltage drop now across the conductive lamp 17 is substantially lower than its previous open circuit voltage drop, and this conductive voltage drop is predetermined to be lower than the breakdown voltage of the trigger diode 32. Since the capacitor 31 only charges until its voltage equals the voltage drop across the conductive lamp 17, the trigger diode 32 is no longer biased into conduction. Therefore, the charge on the capacitor 31 is not discharged through the control electrode-cathode path of the SCR 23, and the SCR 23 is not triggered into condition. The capacitor 22 thus ceases to be periodically discharged through the primary winding of the transformer 27. Consequently, no further discharges occur at the spark gap 34 and no further ignition pulses are transmitted through the igni ted lamp 17 Should the lamp 17 thereafter be extinguished, for example as the result of a brief power failure, the voltage drop across the lamp 17 will again increase to equal the potential difference between the leads 9 and 10 when the power is restored. The ignition cycle recited above will then be repeated, thereby to reignite the arc discharge lamp 17 Thus, there has been provided, in accordance with the present invention, an ignition circuit suitable for use as a lamp start circuit. The ignition circuit further is characterized by being operative to initiate conduction in an arc discharge lamp at the first switching on of the lamp, as reinitiating conduction in the lamp after brief extinguishments. The ignition circuit also eliminates the generation of unwanted spark discharges after the lamp 17 has been ignited.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a circuit for supplying D.C. power to an are discharge lamp characterized in that a relatively high voltage drop appears across said lamp whenever said lamp is nonconductive and a relatively low voltage drop appears across said lamp whenever said lamp is conductive, the combination with said circuit of the improvement of an ignition circuit comprising:

a transformer having a primary and a secondary windfirst and second electrical terminals for connection to a D.C. voltage source;

a capacitor connected in series with said primary winding, said series-connected capacitor and primary winding being connected, respectively, between said first and second terminals whereby said capacitor is charged whenever a D.C. voltage is applied across said first and second electrical terminals;

a spark gap connected in series with said secondary winding, said series-connected spark gap and secondary winding being connected across the electrodes of said lamp whereby to impress across said lamp an ignition pulse for initiating conduction therein whenever a spark discharge is produced at said spark gap;

selectively operable means defining a discharge path for discharging said capacitor through said primary winding; and

means responsive to said voltage drop across said lamp for closing said discharge path when said voltage drop is high and for opening said discharge path when said voltage drop is low whereby said discharge path is closed and said capacitor is discharged through said primary winding to induce an ignition pulse in said secondary winding whenever said lamp is nonconductive thereby to produce a spark discharge at said spark gap and initiate conduction in said lamp.

2. The invention recited in claim 1 wherein said means for defining a discharge path comprises a voltage actuated switch means for short-circuiting the current path across said series-connected capacitor and primary winding whenever said lamp is nonconductive.

3. The invention recited in claim 2 wherein:

said means responsive to said voltage drop across said lamp includes a resistor and a second capacitor seriesconnected across the electrodes of said lamp whereby 6 a control voltage is continuously generated on said riodically transmitted to said control electrode second cgpaiitor by said voltage drop across said 3113 saidl ddischarge path tiffineld by said anode amp; an w erein: an cat 0 e is periodica y cosed W en said said switch means includes a control rectifier and lamp is nonconductive.

a trigger diode, said control rectifier having an 5 anode, cathode and a control electrode, said ferences Cited angdi:J and cathode gefinirilg settid dtilscharige path UNITED STATES PATENTS an -e1ng connece 1n s un W1 sai seriesconnected first capacitor and primary winding, f 1

said trigger diode being connected between said 10 3045148 7/1962 F??? 3 83 control electrode and the junction of said resis- 3189789 6/1965 i et a 1 tor and said second capacitor, said trigger diode owe 3 5 9 having a breakdown voltage of a predetermined magnitude which is periodically exceeded by said control voltage continuously generated on 15 said second capacitor when said lamp is non- 317 96 conductive whereby said control voltage is pe- VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R. 

